Graft versus host disease (GvHD) represents a major complication following bone marrow transplantation (BMT). Even for patients transplanted from an HLA-identical sibling exists a 33% risk of grade II to IV GvHD which is mostly attributed to minor histocompatibility antigen (mHAG) mismatches.Because monocyte-derived dendritic cells (Mo-DC) are easy to culture in vitro, on the one hand, and are as efficient as blood DCs in initiating primary T cell responses, on the other hand; here we have investigated whether Mo-DC could initiate allogeneic T cell responses between HLA genotypically identical siblings. With this aim, as little as 40 ml of blood from sibling of an HLA-identical pair were used and purified monocytes were cultured with GM-CSF and IL-4 for 6 days together with TNFα at the seventh day. The differentiation of monocytes into DCs was documented by CD83 staining, by downregulation of CD14 and by high expression of CD80, CD86, CD40 and HLA-DR, -DQ, and -DP molecules. Mixed lymphocyte cultures were performed in the presence of Mo-DCs as stimulators, or mononuclear cells as control. As expected from the use of DCs, a weak autologous proliferation was induced. However, the positive and significant mixed lymphocyte reaction that was observed predicts that Mo-DC should be an efficient tool for detection of minor Histocompatibility antigen disparities, in vitro.